There have been several devices or instruments available for measuring rotational speeds, for example, a mechanical tachometer, or an electric encoder, etc. Also, as a means contained within an apparatus, a combination of a magnet piece secured to a rotary member and a hall element for detecting the rotation of the magnet piece has been available. Further, as an instrument for measuring the number of revolutions, water meters or distance meters for automobiles, etc. have been available.
However, the above reference means or instruments require a certain space for installation or a complex transmitting means. For example, where a tachometer or encoder is employed, such device has to be coupled to a rotating shaft and a part of such means is required to be secured to an end of the shaft. When a hall element is used to detect the rotation of the magnet piece, it is necessary to secure the magnet piece to the rotary member and, sometimes, such as when the rotation speed is quite high, it is necessary to embed the magnet piece in the rotating member. If a water meter is employed, a gear train involving plural gears would be needed to transmit the rotation of an impeller to a counter which indicates the total sum of the number of revolutions by using each of the figures and this necessitates an expensive and complex one. Also with a distance meter for an automobile, the construction thereof is complex since it employs means for taking up figures.
There has also been available an optical means for detecting an intermittently reflected light from a rotating object. In this case, a rotating object is specifically made transparent or a reflective portion is provided in the rotating object for generating intermittent light pulses.
Thus, the particular arrangements heretofore available for utilizing an optical measuring means have required additional elements and expense.